The present invention relates to an inverter operating in a pulse width modulation (PWM) principle or to a modulation system in the PWM inverter apparatus.
In a conventional PWM inverter, the control is effected by using a so called square wave shaped voltage obtained by chopping certain dc voltage so that the waveform of the chopped current may differ at each of the PWM period and accordingly the value of the commutating current varies greatly depending on the pulse series even under a same load condition. More especially, the duration of one cycle of the inverter is prolonged in counter proportion to the output frequency of the inverter in low speed frequency range. In view of this fact, the number of triangle wave signal forming the carrier signal in this duration or the pulse mode is generally increased at lower speed and thus the peak current at current commutation is suppressed. In this case, the waveform of the input current for the inverter assumes equivalently the same envelope with the motor current formed essentially by RL circuit and increases exponentially so that the current of the pulse voltage at each termination period of the repetition pulses increases also exponentially even the aforementioned increase of the pulse mode. Due to this fact the thyrister element and the current commutating circuit elements forming the inverter device should be selected to withhold the maximum current. The frequency components contained in the inverter input current include high frequency current component of 6 times higher harmonic component of the inverter output frequency beside the abovementioned chopping frequency, since the input current flows from the feeder through a filter in a form of three-phase full-wave rectification of the motor current in general by the inverter. By this reason, at the low frequency time such as the starting time, the feeder current may not be sufficiently smoothed by lowering the filter efficiency so that the feeder current may contain ripple component. Moreover in the rail, in which the inverter input current flows, there is also flow of security signal current which is in general in the commercial frequency. Accordingly, when an electric train is driven by a PWM inverter, the frequency range of the security signal system passes the device in the low frequency range especially at the starting time since the filtering effect is decreased at such low frequency range and certain signal trouble might be caused.